Method of Constructing Inductors and Transformers

ABSTRACT

An embodiment of the invention relates to an apparatus including a magnetic device and a related method. A multilayer substrate is constructed with a winding formed in a metallic layer, an electrically insulating layer above the metallic layer, and a via formed in the electrically insulating layer to couple the winding to a circuit element positioned on the multilayer substrate. A depression is formed in the multilayer substrate, and a polymer solution, preferably an epoxy, containing a ferromagnetic component such as nanocrystaline nickel zinc ferrite is deposited within a mold positioned on a surface of the multilayer substrate above the winding and in the depression. An integrated circuit electrically coupled to the winding may be located on the multilayer substrate. The multilayer substrate may be a semiconductor substrate or a printed wiring board, and the circuit element may be an integrated circuit formed on the multilayer substrate.

This is a continuation application of U.S. application Ser. No.12/262,816, filed on Oct. 31, 2008, entitled “Method of ConstructingInductors and Transformers,” which application is hereby incorporatedherein by reference in its entirety.

TECHNICAL FIELD

An embodiment of the invention relates generally to constructingmagnetic devices, and in particular, to forming a magnetic device suchas an inductor or a transformer on a substrate, such as a printed wiringboard or a semiconductor substrate.

BACKGROUND

With the increasing complexity and level of integration of electronicproducts, there is a growing need for distributed and independent powerconversion devices, such as point-of-load voltage sources, to providethe well-regulated bias voltages for the highly integrated semiconductordevices that are commonly used. Highly integrated semiconductor devicesfrequently operate from specialized bias voltages. The power conversiondevices must be economical and formed with very small dimensions to meetthe size and portability needs of these markets, particularly marketsthat include portable and compact products such as cellular telephonesand personal computers.

A power converter is conventionally formed with discrete magneticdevices such as transformers and inductors that are necessary in thedesign to achieve high power conversion efficiency. Such magneticdevices generally consist of electrically conductive windings, a body(“bobbin”) to support the windings, and a ferromagnetic core to providea sufficiently high level of magnetic flux density for a given level ofcurrent in the windings. The assembly is generally mounted on a printedwiring board (“PWB”) for interconnection with other circuit elements.

A known technique to form magnetic devices is to employ planar windingsthat are formed directly in the buried metallic layers of a PWB.Exemplary magnetic cores that can be used with such structures are “EI”and “EE” core forms, so named for the corresponding shapes of theletters “E” and “I,” and produced by companies such as EPCOS andPhilips. To insert and secure such cores to a PWB, apertures withcomplex and precise shapes must be milled in the PWB. Milling of suchapertures in a PWB is generally a more costly mechanical operation than,for example, drilling of holes.

Power converters are also manufactured with discrete magnetic devices inan integrated circuit-size (“IC”) package, such as the DCR010505 powerconverter produced by Texas Instruments, Inc., to achieve a smallphysical size, as described by Geoff Jones in the paper entitled“Miniature Solutions for Voltage Isolation,” Texas Instruments' AnalogApplications Journal, dated 3Q2005, pages 13-17. However, such powerconverters are generally larger than the needs of the more challengingmarkets for compact circuit devices, particularly for low powerapplications, and are produced at costs that do not meet the needs ofhigh-volume production.

To produce inductors and transformers with small dimensions, studieshave been undertaken to incorporate buried copper conductors withinsurrounding layers of a ferromagnetic film, for example, the studydescribed by K. Yamaguchi, et al., in the paper entitled “Characteristicof a Thin Film Microtransformer with Circular Spiral Coils,” publishedin the IEEE Transaction on Magnetics, Vol. 29, No. 5, Sep. 1993. Thefabrication procedure described by Yamaguchi includes depositing asputtered magnetic film that is then patterned using a photoresist.Copper windings are deposited by an electroplating process to produce acompact magnetic design. However, the overall process is not practicalfor a high-volume, low-cost production sequence in view of thecomplexity of the manufacturing steps that are necessary to produce aworkable product.

Thus, there is a need for a process and related method to produce amagnetic device with very small physical dimensions that are adaptableto high volume and low cost manufacturing processes to meet the morechallenging market needs that lie ahead that avoids the disadvantages ofconventional approaches.

SUMMARY OF THE INVENTION

In accordance with an exemplary embodiment, an apparatus including amagnetic device and a related method are provided. In an embodiment, amultilayer substrate is constructed with a first winding formed in afirst metallic layer of the multilayer substrate, a first electricallyinsulating layer formed above the first metallic layer, and a first viaformed in the first electrically insulating layer. The first via couplesthe first winding to a circuit element positioned on the multilayersubstrate. A depression is formed in the multilayer substrate, and apolymer solution containing a ferromagnetic component is deposited on asurface of the multilayer substrate above the first winding and in thedepression. The polymer solution is preferably an epoxy, but anotherpolymer solution may be used. The ferromagnetic component preferablycontains nanocrystaline nickel zinc ferrite, but another ferromagneticmaterial may be used. In an embodiment, the depression incompletelypenetrates the multilayer substrate. Preferably, the polymer solution isdeposited within a mold positioned on a surface of the multilayersubstrate to form the shape of the polymer solution after curing. Themultilayer substrate may be a printed wiring board. In an embodiment, anintegrated circuit is located on the multilayer substrate that iselectrically coupled to the first winding. In an embodiment, theapparatus may be formed as a power conversion device. In a furtherembodiment, a second insulating layer is formed on the multilayersubstrate below the first metallic layer, and a second winding of themagnetic device is formed in a second metallic layer of the multilayersubstrate to form additional turns for a winding of the magnetic device.The second metallic layer is formed on the multilayer substrate belowthe second insulating layer, and a second via is formed in the secondinsulating layer. The second via electrically couples the second windingto the first winding. Preferably, the first via and the second via aremetallic vias. In a further embodiment, a third insulating layer isformed on the multilayer substrate below the second metallic layer, anda third metallic layer is formed on the multilayer substrate below thethird insulating layer. The third metallic layer forms a further windingof the magnetic device dielectrically insulated from the first windingto form dielectrically insulated transformer windings. A third via isformed in the third insulating layer to provide an electrical couplingof the further winding to a further circuit element located on themultilayer substrate. In an embodiment, the multilayer substrate is asemiconductor substrate, and the circuit element is an integratedcircuit formed on the multilayer substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims. In the figures, identicalreference symbols generally designate the same component partsthroughout the various views, and may be described only once in theinterest of brevity. For a more complete understanding of the invention,reference is now made to the following descriptions taken in conjunctionwith the accompanying drawings, in which:

FIGS. 1A, 1B, and 1C illustrate planar-view drawings showing,respectively, the form and arrangement of spiral, planar coils in anupper metallic layer, a lower metallic layer, and the placement of thewindings relative to each other, constructed according to an embodiment;

FIG. 2 illustrates a planar-view drawing of an electrical apparatusincluding a magnetic device formed with spiral windings formed in atwo-sided PWB, constructed according to an embodiment;

FIG. 3 illustrates an elevation-view drawing of a planar transformerformed on a four-layer PWB, constructed according to an embodiment;

FIG. 4 illustrates an elevation-view drawing showing the use of molds toshape magnetic layers deposited about planar windings, constructedaccording to an embodiment;

FIG. 5 illustrates an elevation-view drawing showing the structure of amagnetic device wherein a magnetic layer is formed on a semiconductorsubstrate, constructed according to an embodiment;

FIG. 6 illustrates a transformer formed on a semiconductor substrateincluding a primary winding formed on metallic layers that aredielectrically isolated from a secondary winding formed on metalliclayers, constructed according to an embodiment; and

FIG. 7 illustrates a simplified schematic diagram of a switch-mode powerconverter including an exemplary magnetic circuit element, constructedaccording to an embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to exemplaryembodiments in a specific context, namely a magnetic device formed on asubstrate with planar windings that produce a magnetic field enhanced bya molded ferromagnetic structure.

An embodiment of the invention may be applied to various electronicpower conversion arrangements, for example, to an integrated isolated ornonisolated power converter configured to power an electronic load.Other devices including a magnetic element can be constructed andapplied using processes as introduced herein in different contexts usinginventive concepts described herein, for example, a filteringarrangement used to shape a spectral characteristic of an analog signal.

As introduced herein, windings of a compact, integrated magnetic devicesuch as a transformer or an inductor are formed on a substrate such as aPWB or a semiconductor substrate. The windings are structured so that noadditional electrically conductive lines or other interconnections suchas wire bonds are needed to interconnect the metallic layers forming theinternal windings of the magnetic device, enabling thereby theproduction of low-profile structures. To extend the magnetic path of theferromagnetic layers in various areas of the spiral coils forming thewindings, a number of drilled holes and trenches that may not penetratethe substrate are formed. The winding areas are then covered with apolymer solution that contains a soft ferrite such as the nanocrystalineferrite FeSiBCuNb or nanocrystaline nickel zinc ferrite to form themagnetic core of the medic device. The polymer solution is of asufficiently low viscosity so that the formed depressions, i.e., theholes and trenches, are readily filled. An exemplary polymer solutionsuch as I-8606M is available from Asahi Kasei, and an exemplary powderedferrite such as Vitroperm is available from Vacuumschmelze GmbH. Therelative permeability value and the saturation flux density of theferrite-mold compound are dependent on the powder filling content. For alow viscosity polymer, a suitable filling fraction, without limitation,is 60% to 85% of the polymer volume; the relative permeability of themolded material may typically vary from 10 to 40 for the cited powderfraction. The saturation flux density can be expressed according toKelly, A. W, et al., as described in the paper entitled“Plastic-Iron-Powder Distributed-Air-Gap Magnetic Material,” IEEE PowerElectronics Specialists Conference, 11 Jun. 1990, pages 25-34, by theequation:

B _(sat) =B _(sat-fill)[x+(1−x)ρ_(pol)/ρ_(fill)],  [1]

where

B_(sat-fill)=saturation flux density of the filler (ferrite powder)

x=filler fraction by polyimide volume

ρ_(pol)=mass density of the polyimide

ρ_(fill)=mass density of the ferrite powder

The required properties of the magnetic core for optimal energyconversion are influenced by the number of vias at the periphery of thewinding and by the thickness of the magnetic layer. For economicalimplementation of the magnetic circuit, the vias in the middle area ofthe magnetic device and at the level of winding may be advantageouslyformed of the same size. The diameter and number of vias formed tocontain the polymer solution should be selected in view of expectedcurrent levels in the magnetic device to manage saturation of themagnetic material using analytical techniques well known in the art. Thesum of the areas of the outer vias is preferably equal to the area ofthe large via formed in the middle of the magnetic device.

Turning now to FIGS. 1A, 1B, and 1C, illustrated are drawings showing,respectively, the structure and arrangement of spiral, planar coils inan upper metallic layer, a lower metallic layer, and the placement ofthe windings relative to each other to provide efficient utilization ofboard area on the internal metallization layers forming a magneticdevice, constructed according to an embodiment. Although round spiralstructures for the windings are illustrated in the figures herein, otherwinding structures may be employed within the broad scope of the presentinvention, such as hexagonal spiral structures. The spiral coils of theupper 3 and the lower 5 spiral metal layers are laid out with oppositegeometric winding sense. The inner ends of the upper coil 3 and lowercoil 5 are coupled by via 18. This creates a stacked winding, whereineach layer produces a magnetic flux with the same winding sense withrespect to flux generation. No other connection is necessary tointerconnect the inner end terminal of each winding. This creates acompact, low-profile, planar coil, wherein a higher number of windingturns is produced compared to a single-layer winding arrangement.Accordingly, no connection is needed to form the mid-point (or other taplocation) of the winding. The coil ends 15 and 16 are coupled to otherelectrical circuits, for example, to the integrated circuit IC1illustrated and described hereinbelow with reference to FIGS. 2 and 3.

Turning now to FIG. 2, illustrated is a planar-view drawing of anembodiment of an electrical circuit including a magnetic device formedwith spiral windings formed in a two-sided or multilayer PWB 201. Theinterconnection 15 and 16 between the windings and an IC (IC1) placed onthe PWB are shown. The windings are covered with a ferromagnetic layer101. The ferromagnetic layer 101 fills the inner via 14 and the outervias such as the outer via 13. The ferromagnetic layer 101 is created byapplying a polymer solution such as an epoxy or other plastic materialmixed with a ferromagnetic powder, as described previously hereinabove.

Turning now to FIG. 3, illustrated is an elevation drawing of anembodiment of a planar transformer 302 formed in a four-layer PWB 301.The transformer consists of the following layers and structures:

layers 101 and 1011: ferromagnetic layers,

layer 102: isolation layer (the first passivation layer above the firstmetallic layer of the first winding),

layer 103: first metallic layer of the first winding,

layer 104: isolation layer between the first and the second metalliclayers of the first winding,

layer 105: second metallic layer of the first winding,

layer 106: substrate/isolation layer of the PWB,

layer 107: second metallic layer of the second winding,

layer 108: isolation layer between the first and the second metalliclayers of the second winding,

layer 109: first metallic layer of the second winding,

layer 1010: isolation layer (the second passivation layer below thefirst metallic layer of the second winding,

12 and 18: vias between upper and lower metallic layers of the first andsecond windings,

13: outer completely or incompletely drilled via through PWB,

14: inner completely or incompletely drilled via through PWB, and

15 and 16: ends forming the interconnections with the first winding.

In order not to saturate the ferrite layer in the area of the via 13,the viscosity of the solution is preferably selected so that it will bedisposed in the region of the via 13 with a thickness that is no lessthan a certain amount such as the distance D indicated in FIG. 3. Thedistance D is selected to manage saturation of the magnetic layer;preferably, it is not less than the diameter of via 13. In order toproduce increased electrical breakdown voltage between the windings, theholes are not drilled completely through the PWB, as indicated by theseparation distance d in FIG. 3. The distance d can also be used tocontrol the effective transformer air gap for the path of the magneticflux.

Turning now to FIG. 4, illustrated is an elevation drawing showing anembodiment of the use of molds to shape and control dimensions of themagnetic layers about the planar windings. As illustrated in FIG. 4, thewindings of the magnetic device are coupled to integrated circuits IC1and IC2, such as by winding end 15 that may be formed with a via. A moldfor the polymer solution, illustrated as the mold with top and bottomportions identified with reference designation 19 with apertures 20, isplaced above the upper and lower surfaces of the PWB 401. The polymersolution containing the ferromagnetic component is introduced throughthe apertures 20 under pressure to fill the mold and the vias in thePWB.

By selection of the height D and the diameter diam of the mold, themagnetic and electrical characteristics of the core can be determined.If the polymer solution containing the ferromagnetic component iscontrollably introduced through the vias 14 and 18, the structure of theresulting ferrite core can be constructed with repeatable magnetic andelectrical characteristics.

A magnetic device as introduced herein can be characterized, withoutlimitation, as follows: It is not necessary for the magnetic flux of awinding to be completely enclosed in a magnetic layer to obtain a highquality inductor with sufficiently high inductance values for aparticular application. A portion of the magnetic flux may lie in air orin other non-ferromagnetic material. The effective permeability of themagnetic path will be increased nonetheless by the presence of theferromagnetic material, which results in increased inductance of aplanar inductor winding formed in the magnetic device. The longer thepath of the magnetic flux lies in a ferromagnetic layer, the greater isthe resulting device inductance. In this way, an inductance value and apower density of a magnetic device formed with a planar winding can besubstantially increased over a comparable device formed without addedferromagnetic material.

Turning now to FIG. 5, illustrated is an elevation drawing showing anexemplary embodiment of the structure of a magnetic device, wherein amagnetic layer 101 is formed on a semiconductor substrate 21. Themagnetic device is coupled to an integrated circuit 501 by means of viasformed through the isolation layers of the semiconductor device. Unlikethe structure of the magnetic devices illustrated in FIGS. 4 and 5, themagnetic flux of the device illustrated in FIG. 6 is less containedwithin a high permeability magnetic layer, such as the magnetic layers101 and 1011 illustrated in FIGS. 3 and 4, but traverses longer,low-permeability paths, such as the paths between the trenches 13 and14. Nonetheless, a sufficiently high permeability path can be obtainedto construct a practical high-frequency magnetic device. The trenches 13and 14 are etched using integrated circuit etching techniques well knownin the art, such as, without limitation, by deposition of a photoresistlayer, patterning the photoresist layer, reactive-ion etching, anddeposition of a metallic layer on the walls of holes and trenches soformed, in the processing steps after forming layers 102 through 106 onthe semiconductor substrate 21. Then the magnetic layer 101 is depositedand structured. The deposition of a thin magnetic layer can be performedemploying RF sputtering in a PVD (plasma vacuum deposition) process.Structuring of the layer can by realized by means of a photoresistlift-off method. Another method is direct deposition of a photoresistcontaining a magnetic powder to fill the trenches and to build the uppermagnetic layer. The thickness of the layer may be controlled by therotational speed of the wafer. Structuring of the photoresist thenfollows. The remaining photoresist structure is preferably not ashed.Instead of the last photoresist, the polyimide containing the magneticpowder can be used. In this way a magnetic circuit is created wherein amagnetic field surrounds a compact, planar winding, and the magneticfield is partially conducted within a high permeability ferromagneticstructure. The magnetic field lines that are conducted between thetrenches such as trenches 13 and 14 are conducted through the isolationlayers 104 and 106 and the substrate 21 of a semiconductor device. It isalso possible to etch vias 13 and 14 entirely though the substrate. Inthis case, the coupling factor between the primary and secondary windingbecomes larger, as well as the primary winding inductance.

Turning now to FIG. 6, illustrated is an exemplary embodiment of atransformer formed on semiconductor substrate 21 including a primarywinding formed on metallic layers 103 and 105 that is dielectricallyisolated from a secondary winding formed on metallic layers 107 and 109.As illustrated in FIG. 5, the magnetic device may be coupled by means ofvias formed through the isolation layers of the semiconductor device toan integrated circuit formed on the substrate. Similar to the magneticdevice illustrated in FIG. 5, the magnetic flux of the deviceillustrated in FIG. 6 is only partially contained within a highpermeability magnetic layer.

Referring now to FIG. 7, illustrated is a simplified schematic diagramof a switch-mode power converter 700, including a magnetic circuitelement L_(filter), constructed according to an embodiment. Theswitch-mode power converter 700, a power conversion device, is anexemplary application of a low-profile magnetic element such as theinductor L_(filter) coupled to circuit elements on a PWB or on anothersubstrate such as a semiconductor substrate. The power converterincludes a controller 701 constructed as an integrated circuit thatregulates the power converter output voltage. The power converterprovides dc power to a load R_(load) (not shown) that may be coupled tooutput terminals 703 and 704. While the illustrated power converteremploys a buck converter topology, those skilled in the art shouldunderstand that other power converter topologies are well within thebroad scope of the present invention.

The power converter receives an input dc voltage V_(input) from a sourceof electrical power 702 at an input thereof and provides a regulatedoutput voltage V_(output) at output terminals 703 and 704. In keepingwith the principles of a buck converter topology, the output voltageV_(output) is generally less than the input voltage V_(input) such thata switching operation of the power switch Q can regulate the outputvoltage V_(output).

During a first portion of a high-frequency switching cycle of the powerconverter, the power switch Q, which may be formed as a power MOSFET, isenabled to conduct in response to a gate drive signal G_(D), couplingthe input voltage V_(input) to the filter inductor L_(filter), enablingthereby a current to flow through the filter inductor L_(filter). Duringthe first portion of the high-frequency switching cycle, an inductorcurrent flowing through the output filter inductor L_(filter) increasesas current flows from the input to the output of the power train. An accomponent of the inductor current is filtered by the output filtercapacitor C_(filter).

During a complementary portion of the switching cycle, the power switchQ is transitioned to a non-conducting state, and a freewheeling diodeD_(fr) coupled to the filter inductor L_(filter) becomes forward biased.The diode D_(fr) provides a current path to maintain continuity ofinductor current flowing through the filter inductor L_(filter). Duringthe complementary portion of the switching cycle, the inductor currentflowing through the filter inductor L_(filter) decreases. In general,the first portion of the high-frequency switching cycle of the powerswitch Q may be adjusted to regulate the output voltage V_(output) ofthe power converter.

The controller 701 of the power converter receives the output voltageV_(output) of the power converter and a desired system voltage. Thecontroller 701 adjusts the first portion of the high-frequency switchingcycle to regulate the output voltage V_(output) at the desired systemvoltage.

The concept has thus been introduced of constructing a multilayersubstrate with a first winding of a magnetic device formed in a firstmetallic layer of the multilayer substrate, a first electricallyinsulating layer formed above the first metallic layer, and a first viaformed in the first electrically insulating layer. The first via couplesthe first winding to a circuit element positioned on the multilayersubstrate. A depression is formed in the multilayer substrate, and apolymer solution containing a ferromagnetic component is deposited on asurface of the multilayer substrate above the first winding and in thedepression. In an embodiment, the polymer solution is an epoxy. In anembodiment, the ferromagnetic component contains nanocrystaline nickelzinc ferrite. In an embodiment, the depression incompletely penetratesthe multilayer substrate. In an embodiment, the polymer solution isdeposited within a mold positioned on a surface of the multilayersubstrate to form the shape of the polymer solution after curing. In anembodiment, the multilayer substrate comprises a printed wiring board.In an embodiment, an integrated circuit is located on the multilayersubstrate that is electrically coupled to the first winding. In anembodiment, the apparatus is a power conversion device. In a furtherembodiment, a second insulating layer is formed on the multilayersubstrate below the first metallic layer, and a second winding of themagnetic device is formed in a second metallic layer of the multilayersubstrate. The second metallic layer is formed on the multilayersubstrate below the second insulating layer, and a second via is formedin the second insulating layer. The second via electrically couples thesecond winding to the first winding. In an embodiment, the first via andthe second via are metallic vias. In a further embodiment, a thirdinsulating layer is formed on the multilayer substrate below the secondmetallic layer, and a third metallic layer is formed on the multilayersubstrate below the third insulating layer. The third metallic layerforms a further winding of the magnetic device electrically insulatedfrom the first winding. A third via is formed in the third insulatinglayer to provide an electrical coupling of the further winding to afurther circuit element located on the multilayer substrate. In anembodiment, the multilayer substrate is a semiconductor substrate, andthe circuit element is an integrated circuit formed on the multilayersubstrate.

Another exemplary embodiment provides a method of forming an apparatus.In an embodiment, the method includes forming a first metallic layer ofa multilayer substrate, and forming a first winding of a magnetic devicein the first metallic layer. The method further includes forming a firstelectrically insulating layer above the first metallic layer, andpositioning a circuit element on the multilayer substrate. The methodfurther includes forming a first via in the first electricallyinsulating layer to couple the first winding to the circuit element, andforming a depression in the multilayer substrate. The method furtherincludes depositing a polymer solution containing a ferromagneticcomponent on a surface of the multilayer substrate above the firstwinding and in the depression. In an embodiment, the polymer solutionincludes an epoxy. In an embodiment, the ferromagnetic componentincludes nanocrystaline nickel zinc ferrite. In an embodiment, thedepression incompletely penetrates the multilayer substrate. In anembodiment, the method further includes positioning a mold on a surfaceof the multilayer substrate and depositing the polymer solution in themold. In an embodiment, the multilayer substrate comprises a printedwiring board. In an embodiment, the apparatus comprises a powerconversion device. In an embodiment, the method further includes forminga second insulating layer on the multilayer substrate below the firstmetallic layer, and forming a second metallic layer on the multilayersubstrate below the second insulating layer. The method further includesforming a second winding of the magnetic device in the second metalliclayer, and forming a second via in the second insulating layer toelectrically couple the second winding to the first winding. In anembodiment, the first via and the second via are metallic vias. In anembodiment, the method further includes forming a third insulating layeron the multilayer substrate below the second metallic layer and forminga third metallic layer on the multilayer substrate below the thirdinsulating layer. The method further includes forming a further windingof the magnetic device in the third metallic layer electricallyinsulated from the first winding, and locating a further circuit elementon the multilayer substrate. The method further includes forming a thirdvia in the third insulating layer to provide an electrical coupling ofthe further winding to the further circuit element. In an embodiment,the multilayer substrate is formed as a semiconductor substrate, and thecircuit element is an integrated circuit formed on the multilayersubstrate.

Although processes for forming a device containing a magnetic elementand related methods have been described for application to electronicpower conversion, it should be understood that other applications ofthese processes such as for analog signal filtering are contemplatedwithin the broad scope of the invention, and need not be limited toelectronic power conversion applications employing processes introducedherein.

Although the invention has been shown and described primarily inconnection with specific exemplary embodiments, it should be understoodby those skilled in the art that diverse changes in the configurationand the details thereof can be made without departing from the essenceand scope of the invention as defined by the claims below. The scope ofthe invention is therefore determined by the appended claims, and theintention is for all alterations that lie within the range of themeaning and the range of equivalence of the claims to be encompassed bythe claims.

In the claims:
 1. An apparatus, comprising: a multilayer substratecomprising a first winding of a magnetic device formed in a firstmetallic layer of the multilayer substrate; a first electricallyinsulating layer formed above the first metallic layer; a first viaformed in the first electrically insulating layer, wherein the first viais configured to couple the first winding to a connection for a circuitelement positioned on the multilayer substrate; a depression formed inthe multilayer substrate; and a ferromagnetic material disposed on asurface of the multilayer substrate above the first winding and in thedepression, wherein the first winding is not completely encircled by theferromagnetic material.
 2. (canceled)
 3. The apparatus as claimed inclaim 1, wherein the ferromagnetic material comprises nanocrystalinenickel zinc ferrite.
 4. The apparatus as claimed in claim 1, wherein thedepression incompletely penetrates the multilayer substrate. 5.(canceled)
 6. The apparatus as claimed in claim 1, wherein themultilayer substrate comprises a printed wiring board.
 7. The apparatusas claimed in claim 1, further comprising an integrated circuit locatedon the multilayer substrate and electrically coupled to the firstwinding.
 8. The apparatus as claimed in claim 7, wherein the apparatuscomprises a power conversion device.
 9. The apparatus as claimed inclaim 1, further comprising: a second insulating layer formed on themultilayer substrate below the first metallic layer; a second winding ofthe magnetic device formed in a second metallic layer of the multilayersubstrate, wherein the second metallic layer is formed on the multilayersubstrate below the second insulating layer; and a second via formed inthe second insulating layer, wherein the second via electrically couplesthe second winding to the first winding.
 10. The apparatus as claimed inclaim 9, wherein the first via and the second via are metallic vias. 11.The apparatus as claimed in claim 9, further comprising: a thirdinsulating layer formed on the multilayer substrate below the secondmetallic layer; a third metallic layer formed on the multilayersubstrate below the third insulating layer, wherein the third metalliclayer forms a further winding of the magnetic device electricallyinsulated from the first winding; and a third via formed in the thirdinsulating layer, wherein the third via provides an electrical couplingof the further winding to a further connection configured to be coupledto a further circuit element located on the multilayer substrate. 12.The apparatus as claimed in claim 11, wherein the multilayer substratecomprises a semiconductor substrate, and wherein the circuit elementcomprises an integrated circuit formed on the multilayer substrate. 13.A method of forming an apparatus, the method comprising: forming a firstmetallic layer of a multilayer substrate; forming a first winding of amagnetic device in the first metallic layer; forming a firstelectrically insulating layer above the first metallic layer; forming afirst via in the first electrically insulating layer to couple the firstwinding to a connection configured to be coupled to a circuit element;forming a depression in the multilayer substrate; and forming aferromagnetic material on a surface of the multilayer substrate abovethe first winding and in the depression, wherein the first winding isnot completely encircled by the ferromagnetic material.
 14. (canceled)15. The method as claimed in claim 13, wherein the ferromagneticmaterial comprises nanocrystaline nickel zinc ferrite.
 16. The method asclaimed in claim 13, wherein the depression incompletely penetrates themultilayer substrate.
 17. (canceled)
 18. The method as claimed in claim13, wherein the multilayer substrate comprises a printed wiring board.19. The method as claimed in claim 13, wherein the apparatus comprises apower conversion device.
 20. The method as claimed in claim 13, furthercomprising: forming a second insulating layer on the multilayersubstrate below the first metallic layer; forming a second metalliclayer on the multilayer substrate below the second insulating layer;forming a second winding of the magnetic device in the second metalliclayer; and forming a second via in the second insulating layer toelectrically couple the second winding to the first winding.
 21. A powerconverter, comprising: a magnetic circuit element comprising amultilayer substrate comprising a first winding formed in a firstmetallic layer of the multilayer substrate, a first electricallyinsulating layer formed above the first metallic layer, a first viaformed in the first electrically insulating layer, wherein the first viais configured to couple the first winding to a first connection, adepression formed in the multilayer substrate, and a ferromagneticmaterial disposed on a surface of the multilayer substrate above thefirst winding and in the depression, wherein the first winding is notcompletely encircled by the ferromagnetic material; and a switch coupledto the first connection.
 22. The power converter of claim 21, whereinthe ferromagnetic material is a polymer solution containing aferromagnetic component that is deposited on the surface of themultilayer substrate and in the depression.
 23. The power converter ofclaim 21, wherein the magnetic circuit further comprises: a secondinsulating layer formed on the multilayer substrate below the firstmetallic layer; a second winding of the magnetic device formed in asecond metallic layer of the multilayer substrate, wherein the secondmetallic layer id formed on the multilayer substrate below the secondinsulating layer; and a second via formed in the second insulatinglayer, wherein the second via electrically couples the second winding tothe first winding.
 24. The power converter of claim 21, wherein themagnetic circuit comprises an inductor.